Yes, there is a glitch that if you get any DLC achievements before finishing all the single player, you will not be able to unlock that achievement. The way the achievement works, is it scans your last 50 achievements earned and it has to find all your single player achievements there to unlock.

Pants if it scans the last 50 achievements, i have unlocked 39 of 60, all of the single player ones except this and the team 50%+ kill ones BUT including several DLC ones it should work regardless. Because I dont have 51-60 cheves shouldnt it work?

Another words if I had 57 of 60 achievements, it would only scan the 7th through 57th achievements in the order I unlocked them, therefore never seeing the 1st Act, or training ones etc. which everyone has first, and NOT allowing it to unlock. But if you have less than 50, it should work? right?